This invention relates generally to devices for measuring eyes and more particularly to an ophthalmic measuring instrument which can be applied directly to an eyeglass frame selected by the patient so that a monocular measurement of the patient's visual axis can be obtained for each respective eye; and measurement of both the vertical inclination and convergence of the visual axes can be made; all of which measurements are critical in prescribing lenses, telescopes, and microscopes for patients with special optical needs.
When formulating the prescription for corrective lenses, the conventional technique for determining the geometric centers for such corrective lenses is to measure the interpupillary distance (PD) for the patient's eyes. This has been done by the examiner's objective use of a metric ruler or other device for measuring the distance between the inner edge of one pupil and the outer edge of the other pupil of the patient's eyes.
It has been found that scrupulously derived results from the conventional twenty-one point optometric examination can be voided by inaccurate PD measurements for the far point and near point working distances of the patient.
Because of the importance of these measurements to the examiner in determining the proper prescription for the patient's lenses particularly in low vision patients, various devices are known in the prior art for making accurate measurements of the PD distance for a patient's eyes such as is shown in U.S. Pat. Nos. 1,052,161, 2,197,139, and 2,491,312.
In U.S. Pat. No. 2,491,312 the problems which are met and the reasons for such measurements particularly in the filling and providing of bi-focal and multi-focal lenses is set forth in some detail.
While some prior art methods of measurement may be adequate to provide PD measurements for distance vision excepting eccentric fixation, the use thereof for providing the PD measurement for near vision results in errors which can cause induced prism and cylinder, in turn causing asthenopic and even aniseikonic symptoms. Prentice's rule known to those skilled in the art has shown that prism is a function of dioptric power times centemeters of displacement. Therefore, any error in these conventional methods of objective measurement of PD become critical when prescribing a high dioptric value prescription.
In this regard one critical source of error which the conventional methods of objective measurement of PD distance for a patient's eyes does not and cannot account for is the known fact that the visual axis for the eye passes 1 mm. nasal to the geometric axis for the given eye, that is the line passing through the anterior and posterior poles of the eye.
This nasal characteristic of the visual axis is not significant for distance vision because the visual axes are essentially parallel and therefore the conventional methods of objective measurement for the PD of the patient's eyes and the subjective or patient's measurements of the visual axes for the eyes will be essentially in agreement except for eccentric fixation.
When, however, these methods of measurements are applied to near vision, the inter-visual axes distance will be consistantly smaller because of the convergence of the visual axes at the vertex distance of the plane of the eyeglass frame.
Such inaccuracies of the conventional objective measurement techniques can be tolerated for most sphero-cylindrical refractive errors. This is not the case for spherical refractive errors above 4.00 diopters (D) and cylinder above 1.00 D and/or where a patient requires high refractive lenses, telescopes and/or microscopes. If these inaccuracies and errors are present they can cause many vision problems.
The monocular measurement of the patient's visual axis cannot be overlooked for the further reason that optical and facial assymetries can cause the geometric center of the lens for each eye to be displaced from the point at which the visual axes actually passes through the lens. Facial assymetry on each patient is such that the distance between each eye and the bridge of the nose is seldom equal.
In U.S. Pat. No. 2,884,832, a device and method is disclosed for measuring the inter-pupillary axes distance for a given patient's eyes by measuring the monocular pupillary axis distance for each of the respective eyes of the patient and the near convergence of the pupillary axes distance by measuring the inter-pupillary axes distance for the combined or binocular action of both eyes. However, the device utilized with this method of measuring can itself introduce error because it is difficult for the patient and the examiner to set and to maintain with the setting element thereof the vertical position of the visual axis for a given eye being measured, it does not allow vertical or horizontal measurement of the inclination of the visual axis which is important when prescribing spectacle-mounted telescopes and surgical binoculars, and the width of the target card used does not equal the visual angle subtended through the setting or viewing means of the device at the working distances.
The present invention seeks to meet and overcome these problems by providing a new and improved device which permits the examiner to define the visual axis for each of the patient's eyes. This device is affixed to the eyeglass frames selected by the patient so that the center of the eyeglass frame serves as a reference point. By placing the eyeglass frame with the improved device in accordance with the present invention thereon, in position to the patient's face, and having the patient view sized targets for the near working distance therethrough, the patient can subjectively provide the examiner with measurements of; the monocular visual axis (MVA) distance of the respective eyes of the patient relative the center of the eyeglass frame; measurements of the exact vertical displacement of the visual axis for the respective eyes of the patient; and also measurements of the inclination and vergence of the visual axis of each of the patient's eyes particularly for near vision so as to eliminate all possible sources of error and simplify and provide a standardized method for measuring the inter-visual axes for a patient's eyes from which an examiner can more accurately prescribe the required corrective lenses, telescopes and microscopes for patients with special optical needs.